Maxwell and Special RelativityKirk T. McDonald

Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544(May 26, 2014; updated May 5, 2015)

It is now commonly considered that Maxwell’s equations [1] in vacuum implicitly containthe special theory of relativity.1

For example, these equations imply that the speed c of light in vacuum is related by2

c =1√ε0μ0

, (1)

where the constants ε0 and μ0 can be determined in any (inertial) frame via electrostatic andmagnetostatic experiments (nominally in vacuum).3 Even in æther theories, the velocity ofthe laboratory with respect to the hypothetical æther should not affect the results of thesestatic experiments,4 so the speed of light should be the same in any (inertial) frame. Then,the theory of special relativity, as developed in [2], follows from this remarkable fact.

Maxwell does not appear to have crisply drawn the above conclusion, that the speed oflight is independent of the velocity of the observer, but he did make arguments in secs. 599and 770 of [3] that correspond to the low-velocity approximation to special relativity, aspointed out in sec. 5 of [6]. These arguments contrast with claims [7, 8] that Maxwell tacitlyassumed that Galilean relativity [9] applied to his electrodynamics.

The notion of Galilean electrodynamics seems to have been developed only in 1973 [9]. Inthis view there are no electromagnetic waves, and only quasistatic phenomena, so this notionis hardly compatible with Maxwellian electrodynamics. In fact, there are two variants ofGalilean electrodynamics, so-called electric Galilean relativity in which the transformationsbetween two inertial frames with relative velocity v are (sec. 2.2 of [9], but given here inGaussian units)5

ρ′e = ρe, J′e = Je − ρev (c |ρe| � |Je|),

E′e = Ee, B′

e = Be − v

c× Ee fe = ρeEe (electric), (2)

and so-called magnetic Galilean relativity (sec. 2.3 of [9]) with transformations

ρ′m = ρm − v

c2· Jm J′

m = Jm (c |ρe| � |Je|),

E′m = Em +

v

c× Bm, B′

m = Bm fm = ρm

(Em +

v

c×Bm

)(magnetic). (3)

1Maxwell’s electrodynamics was the acknowledged inspiration to Einstein in his 1905 paper [2].2Equation (1) is a transcription into SI units of the discussion in sec. 80 of and sec. 758 of [3].3As discussed in [4], examples of a “static” current-carrying wire involve effects of order v2/c2 where v

is the speed of the moving charges of the current. A consistent view of this in the rest frame of the movingcharges requires special relativity. These arguments could have been made as early as 1820, but it took 85years for them to be fully developed.

It is now sometimes said that electricity plus special relativity implies magnetism, but a more historicalview is that (static) electricity plus magnetism implies special relativity. This theme is emphasized in [5].

4This ansatz is a weak form of Einstein’s Principle of Relativity.5In Galilean electrodynamics the symbol c does not represent the speed of light (as light does exist in

this theory), but only the function 1/√

ε0μ0 of the (static) permittivity and permeability of the vacuum.

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For comparison, the low-velocity limit of special relativity6 has the transformations

ρ′s ≈ ρs −v

c2· Js J′

s ≈ Js − ρsv,

E′s ≈ Es +

v

c× Bs, B′

s ≈ Bs − v

c× Es (special relativity, v � c). (4)

1 Section 599 of Maxwell’s Treatise

In sec. 599 of his Treatise [3], Maxwell argues that an electric charge q which moves withvelocity v in electric and magnetic fields E and B experiences an electromagnetic force

F = q(E +

v

c× B

). (5)

This is now known as the Lorentz force, and it seems seldom noted that Maxwell gave thisform, likely because he disguised it by writing (in electromagnetic units)

E = v × B − ∂A

∂t− ∇ψ, (6)

where E is the “electromotive intensity,” i.e., F/q, A is the vector potential and ψ is thescalar potential. Then, eq. (5) follows noting that the electric field is given (in emu) by−∂A/∂t− ∇ψ, and that v × B in emu becomes v/c× B in Gaussian units.

We can also interpret Maxwell’s E and the electric field E′ in the frame of the movingcharge, such that Maxwell’s transformation of the electric field is

E′ = E +v

c× B (Maxwell, sec. 599 of [3]). (7)

The transformation (7) is compatible with both magnetic Galilean relativity, eq. (3), andthe low-velocity limit of special relativity, eq. (4). These two version of relativity differ as tothe transformation of the magnetic field. In particular, if B = 0 while E were due to a singleelectric charge at rest (in the unprimed frame), then magnetic Galilean relativity predictsthat the moving charge/observer would consider the magnetic field B′ to be zero, whereasit is nonzero according to special relativity.

Maxwell does not comment in sec. 599 on the magnetic field as observed by the movingcharge. He does makes some remarks on the kinematic relations between two frames ofreference with relative velocity v in secs. 600-601, assuming Galilean relativity for these,with the main (and well-known) conclusion being that force is the same in both frames,F′ = F (as is also true in the low-velocity approximation to special relativity).7

2 Sections 769-770 of Maxwell’s Treatise

Faraday considered that a moving electric charge generates a magnetic field [11] (as quotedin [12]). Maxwell also argued for this in secs. 769-770 of [3], where his verbal argument can

6Of course, electromagnetic waves can exist in special relativity, and propagate in vacuum with speed c.7It was noted by Larmor in 1884 on p. 12 of [10] that Maxwell’s analysis also implies effects of order

v2/c2, although Larmor ignored these.

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be transcribed as

B =v

c× E, (8)

for the magnetic field experienced by a fixed observer due to a moving charge. Maxwellnoted that this is a very small effect, and claimed that it had never been observed. How-ever, the magnetic field of a moving charge had been detected in 1876 by Rowland [12, 13](while working in Helmholtz’ lab in Berlin). The form (8) was verified more explicitly byJ.J. Thomson in 1881 [14] for uniform speed v � c, and for any v < c by Heaviside [15] andby Thomson [16] in 1889 (which latter two works gave the full special-relativistic form for Eas well).

If v represents the velocity of a moving observer relative to a fixed electric charge, theneq. (8) implies that the magnetic field experienced by the moving observed would be

B′ = −v

c× E, (9)

This corresponds to the low-velocity limit (4) of special relativity, but not to the prediction(3) of magnetic Galilean relativity (or to the prediction (2) of electric Galilean relativity).It is therefore wrong to argue [7, 8] that Maxwell and his followers (such as Thomson, Heav-iside and FitzGerald [17, 18]) tacitly used Galilean relativity for transformation of the field,although they did use Galilean transformations of spacetime coordinates and mechanicalquantities.

This note was stimulated by e-discussions with Dragan Redzic.

A Appendix: J.J. Thomson (1880)

While secs. 599 and 769-770 of Maxwell’s Treatise are consistent with the low-velocity limitof special relativity, this was not evident at the time, when electromagnetism was generallyinterpreted in an æther theory. For example, in his first research paper, J.J. Thomson [19]used secs. 598-599 of Maxwell’s Treatise to reach a peculiar conclusion as to the speed oflight in a dielectric medium that has velocity v with respect to the frame of the æther.

In the present section, quantities in the ether frame will be unprimed, while a quantityin the frame of the moving dielectric will be denoted with a ′.8

Thomson begins with Maxwell’s eq. (7) and notes that B = ∇ × A where the vectorpotential obeys ∇ · A = 0.

Thomson’s eqs. (1)-(3) correspond to relating the electric displacement field in the movingframe by

D′ = ε0(E′ + P′), (10)

where P′ is the electric polarization field in the moving frame. His eq. (4) is more properlythen

∇′ · D′ = 0. (11)

8The dielectric medium could be vacuum.

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Thomson’s goal is to deduce a wave equation for D′, and to infer from this the speedof light in the moving frame. To this end, he takes the curl of eq. (10), assuming that∇′ × P′ = 0 and that the dielectric medium is in uniform motion such that derivatives ofthe velocity v are zero,

∇′ × D′ = ε0∇′ ×E′ = ∇′ × E + v(∇′ · B) −(v

c· ∇′

)B. (12)

Thomson then supposes that the effect of taking derivatives with respect to spacetimecoordinates x, y, z and t is the same in the ether frame and in the moving frame. That is,he assumes that Galilean relativity relates these coordinates in the two frames. Note thatthis assumption was not needed in the interpretation of Maxwell’s secs. 599 and 769-770(although Maxwell did use this assumption in his secs. 600-601).

With this tacit assumption of Galilean relativity for (x, y, z, t), eq. (12) can be written

∇ × D′ = ∇ ×E + v(∇ · B) −(v

c· ∇

)B = −1

c

∂B

∂t−

(v

c· ∇

)B, (13)

in that ∇ · B = 0 (while in special relativity, ∇′ · B �= 0).It was not appreciated in 1880, and perhaps not until the work of LeBellac and Levy-

Leblond in 1973 [9], that Maxwell’s equations are not consistent with Galilean relativity.When Maxwell’s eq. (7) applies, the appropriate modifications to Maxwell’s equations tobe compatible with Galilean relativity are those of so-called magnetic Galilean relativity(sec. 2.3 of [9]),

∇ · Dm = 4πρm, ∇ · Bm = 0, ∇ × Em = −1

c

∂Bm

∂t, ∇ ×Hm =

4π

cJm (14)

where ρ and J and the volume densities of “free” charge and currents, and there is no“displacement current” and no electromagnetic waves. While the velocity c has a valueequal to the speed of light in vacuum it is to be deduced from static experiments and is notrelated to (nonexistent) wave propagation in Galilean relativity.

Thomson considered a nonmagnetic dielectric medium in which B = H, with no freecharge or current densities, and supposed that in this case

∇× B =1

c

∂D

∂t, (15)

whereas in a consistent Galilean view of this case, ∇ × B = 0. He then took the curl ofeq. (13), writing (his eq. (10))

∇2D′ =1

c2∂D

∂t2−

(v

c2· ∇

)∂D

∂t, (16)

In the magnetic Galilean relativity of [9] this would be just ∇2D′ = 0, corresponding toinstantaneous propagation of electromagnetic effects.

At this point Thomson seems to have assumed that D′ = D even though his argumentbegan with Maxwell’s relation (7) in which E′ �= E. Assuming a wavefunction D′ = D =D0 e

i(kx−ωt) and v = v x, the wave equation (16) leads to the dispersion relation

ω2 − vkω − k2c2 = 0, v′ =ω

k=

1

2

⎛⎝v ± 2c

√1 +

v2

8c2

⎞⎠ . (17)

4

Only the positive root could make sense, leading to9

v′ ≈ c +v

2(v � c), (18)

which was interpreted as the speed of the waves in the moving dielectric (even if that dielectricwere vacuum).

While this result makes no sense from a “modern” perspective, it illustrates how naıveassumptions about relativity and Maxwell’s equations can lead to peculiar conclusions.

References

[1] J.C. Maxwell, A Dynamical Theory of the Electromagnetic Field, Phil. Trans. Roy. Soc.London 155, 459 (1865),http://physics.princeton.edu/~mcdonald/examples/EM/maxwell_ptrsl_155_459_65.pdf

[2] A. Einstein, Zur Elektrodynamik bewegter Korper, Ann. d. Phys. 17, 891 (1905),http://physics.princeton.edu/~mcdonald/examples/EM/einstein_ap_17_891_05.pdf

http://physics.princeton.edu/~mcdonald/examples/EM/einstein_ap_17_891_05_english.pdf

[3] J.C. Maxwell, A Treatise on Electricity and Magnetism, Vol. 2, 3rd ed. (ClarendonPress, 1892), http://physics.princeton.edu/~mcdonald/examples/EM/maxwell_treatise_v2_92.pdf

[4] K.T. McDonald, Charge Density in a Current-Carrying Wire (Dec. 23, 2010),http://physics.princeton.edu/~mcdonald/examples/wire.pdf

[5] D.J. Dunstan, Derivation of special relativity from Maxwell and Newton, Proc. Roy.Soc. London A 366, 1861 (2008),http://physics.princeton.edu/~mcdonald/examples/EM/dunstan_ptrsla_366_1861_08.pdf

[6] G.H. Keswani and C.W. Kilmister, Intimations Of Relativity Relativity Before Einstein,Brit. J. Phil. Sci. 34, 343 (1983),http://physics.princeton.edu/~mcdonald/examples/GR/keswani_bjps_34_343_83.pdf

[7] U. Bartocci and M.M. Capria, Some remarks on classical electromagnetism and theprinciple of relativity, Am. J. Phys. 59, 1030 (1991),http://physics.princeton.edu/~mcdonald/examples/EM/bartocci_ajp_59_1030_91.pdf

[8] D.V. Redzic, Force exerted by a moving electric current on a stationary or co-movingcharge: Maxwell’s theory versus relativistic electrodynamics, Eur. J. Phys. 35, 045011(2014), http://physics.princeton.edu/~mcdonald/examples/EM/redzic_ejp_35_045011_14.pdf

[9] M. LeBellac and J.-M. Levy-Leblond, Galilean Electromagnetism, Nuovo Cim. 14B, 217(1973), http://physics.princeton.edu/~mcdonald/examples/EM/lebellac_nc_14b_217_73.pdf

[10] J. Larmor, Electromagnetic Induction in Conducting Sheets and Solid Bodies, Phil.Mag. 17, 1 (1884), http://physics.princeton.edu/~mcdonald/examples/EM/larmor_pm_17_1_84.pdf

9For sound waves we expect v′ = c−v. That the sign in eq. (18) is “wrong” seems to have gone unnoticed.

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[11] M. Faraday, Experimental Researches, Vol. 1, Art. 1644.

[12] H.A. Rowland and C.T. Hutchinson, On the Electromagnetic Effect of Convection-Currents, Phil. Mag. 27, 445 (1889),http://physics.princeton.edu/~mcdonald/examples/EM/rowland_pm_27_445_89.pdf

[13] H.A. Rowland, On the Magnetic Effect of Electric Convection, Am. J. Sci. 15, 30 (1878),http://physics.princeton.edu/~mcdonald/examples/EM/rowland_ajs_15_30_78.pdf

[14] J.J. Thomson, On the Electric and Magnetic Effects produced by the Motion of Elec-trified Bodies, Phil. Mag. 11, 229 (1881),http://physics.princeton.edu/~mcdonald/examples/EM/thomson_pm_11_229_81.pdf

[15] O. Heaviside, On the Electromagnetic Effects due to the Motion of Electrificationthrough a Dielectric, Phil. Mag. 27, 324 (1889),http://physics.princeton.edu/~mcdonald/examples/EM/heaviside_pm_27_324_89.pdf

[16] J.J. Thomson, On the Magnetic Effects produced by Motion in the Electric Field, Phil.Mag. 28, 1 (1889), http://physics.princeton.edu/~mcdonald/examples/EM/thomson_pm_28_1_89.pdf

[17] G.F. FitzGerald, On Electromagnetic Effects Due to the Motion of the Earth, Trans.Roy. Dublin Soc. 1, 319 (1882),http://physics.princeton.edu/~mcdonald/examples/EM/fitzgerald_trds_82.pdf

[18] K.T. McDonald, Force between Two Comoving Electric Charges (May 24, 2014),http://physics.princeton.edu/~mcdonald/examples/comoving.pdf

[19] J.J. Thomson, On Maxwell’s Theory of Light, Phil. Mag. 9, 284 (1880),http://physics.princeton.edu/~mcdonald/examples/EM/thomson_pm_9_284_80.pdf

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